A hybrid directional coupler is a four port electromagnetic device that is configured to provide an output that is proportional solely to the power incident from a source. For a given bandwidth, a hybrid directional coupler will divide the incident power that is input to one port between two other output ports at quadrature phase. The relative power at each other port with respect to the incident power at the input port will be known based upon the impedances coupled to the various output ports of the device.
Quadrature hybrid directional couplers are used in communications equipment. Such couplers allow a sample of a communications signal that is input at an input port and output at an output or “direct” port, to be taken from the signal at third or “coupled” port. Similarly, there will be no appreciable signal at the fourth or “isolated” port. When appropriately designed, a directional coupler may discern between a signal input at the input port and a signal input at the direct or output port. Such ability to discern the signals is particularly useful when, for example, a coupler is coupled intermediate an RF amplifier and an antenna. In such a configuration, the output of the RF amplifier may be monitored independently from that of a signal reflected from a mismatched antenna. Moreover, such a monitored signal may be used to control the gain (e.g., automatic gain control (AGC)), or reduce the distortion of the RF amplifier.
Directional couplers have been constructed in a variety of different designs. Initially, directional couplers were constructed by sandwiching conductive copper strips or traces between pieces of dielectric material, such as polyolefin or Teflon. Directional couplers were also constructed by locating the inner conductors of two coaxial cables in close proximity with each other, and surrounding them with a common outer conductor. Directional couplers constructed using conductive traces deposited on dielectric materials also included metal containers for housing the dielectrics with, coaxial connectors mounted to the containers to provide connections to the traces. Today such construction techniques are typically used only for high power applications, and may or may not use dielectric materials.
Subsequently, directional couplers were developed without bulky metal housings and coaxial connectors, thereby reducing the size, weight, and cost, and improving the manufacturing of the couplers, as the well as the products using these couplers. These miniaturized directional couplers, often referred to as “filmbrids”, are laminated stripline assemblies that may be bonded together by fusion or by thermoplastic or thermoset films, and are often dispensed from reels and wave soldered onto land areas on circuit boards.
Many ways of constructing directional couplers have been developed; however, practically all of these designs suffer from insertion losses. Insertions losses may be generally attributed to the conductors and dielectric materials used in the construction of many couplers upon which the conductors are deposited, etched or otherwise placed. For example, dielectric materials absorb some of the power applied to a coupler, resulting in throughput or insertion losses. Such losses are particularly troublesome when a coupler is coupled intermediate an RF amplifier and an antenna, since such losses require more amplifier output to overcome the losses inherent in the coupler.
The relative propensity of the dielectric materials in couplers to absorb energy is generally designated by tan(d), the dielectric absorption factor or constant, and is related to air (tan(d)=0). The higher the loss tangent (tan(d)) or loss factor, and the more dielectric material used, the greater the amount of energy absorbed and the greater the losses.
For example, the dielectric absorption factor for pure Teflon (PTFE) is on the order of 0.0006, however pure Teflon is typically unworkable and impractical for use in couplers. As a result, a material, such as fiberglass, may be added to Teflon to provide strength and workability, the dielectric absorption factor there being on the order of 0.001. Other materials typically have dielectric absorption factors on the order of 0.03 or greater. Thus, for a like sized dielectric, a material with a higher dielectric absorption factor will absorb more energy than a material with a lower dielectric absorption factor, resulting in greater insertion loss for a directional coupler constructed using the material with the higher dielectric absorption factor.
Ideally, to minimize losses, a coupler would be constructed with an air dielectric. However, currently available air dielectric couplers have a sheet metal housing, or outer conductor, that is expensive to manufacture and is difficult to surface mount due to co-planarity issues between the housing and attached connections, or leads. Further, the leads are fragile and easily damaged.
Therefore, there is still a need for improving couplers. Particularly, there is a need for a low loss coupling device or coupler that is easy and relatively inexpensive to manufacture and mount.